This invention pertains generally to a noise filter, more specifically to a filter particularly suited for suppressing noise and ripple components in power supply and smoothing circuits.
Noise filters are generally used for preventing malfunctions of electronic equipment by noise transmitted thereto on the power bus superimposed on its power voltage or current, and for suppressing any noise generated on a signal to be transmitted from one piece of equipment to another or to a power supply.
Hitherto, noise filters having an inductor (L) connected in series with a power supply and a shunt capacitor (C), as shown in FIG. 1, have been widely used for the above purposes.
The inductor (L) for use in such noise filter circuits has taken various forms including coils (2) wound on a magnetic core (1), such as a toroidal type ferrite core or a dust core as shown in FIG. 2(a), and conductive wire (5) passing through holes (3) in a bead magnetic core (4) as shown in FIG. 2(b).
A major problem associated with the manufacture of such noise filters is the high production cost of the inductor (L). The pressing and heating processes in the manufacturing of the magnetic core, and the winding of the coil, in addition to the connection of the capacitor (C) with the inductor (L) are particularly cost intensive.
Many attempts have been made to reduce the costs and to thus provide less expensive noise filters. One approach is to integrate both inductive and capacitive elements into a single device which does not require a magnetic core. Japanese laid open patent (Kokai) number 9764/77 to M. Kakutani et al. shows a power supply noise filter made of a metal foil wound in a spiral manner with a pair of lead wires connected at both ends to form the inductor (L). Additionally, another metal foil is laminated with the first mentioned metal foil with an insulting separator therebetween to form the shunt capacitance. Also shown in Japanese utility model publication No. 24900/80 to H. Nakamura et al. is an improvement of such metal foil spiral inductor, namely the addition of external inductors to provide a bypass of DC signal components. These two prior art embodiments address some of the problems associated with the conventional noise filters which include discrete inductors and capacitors, however, they produce a filter having relatively low inductance.
Japanese laid open patent (Kokai) No. 21810/82 to T. Sato et al. discloses a technique to increase the spiral inductance by coating magnetic powder on one or both surfaces of the insulative film. However, this technique requires additional processing and is more expensive than the prior art noise filters.
It is desirable to have a noise filter which is as easy to construct as the wound foil filters with sufficient inductance so that it is not necessary to add external inductors or to employ an expensive wire coating process as in the prior art. The present invention provides such a noise filter.